The present invention relates to an f.theta. lens system in an optical scanner using a rotary polygon mirror.
An optical scanner is known as a device for writing and reading information by scanning a light beam and is used in a laser printer, a facsimile, etc. In such an optical scanner, there is a device having an optically scanning system in which an approximately parallel light beam from a light source is focused and formed as a line image extending in a main scanning corresponding direction and this light beam is deflected at an equal angular velocity by a rotary polygon mirror having a reflecting face in the vincinity of an image forming position of the line image. In this system, the deflected light beam is further focused and formed on a scanned face in the shape of a spot by an image forming lens system to optically scan the scanned face.
In the optical scanner using the rotary polygon mirror, there is a problem about the inclination of the reflecting face. Further with respect to the deflected light beam, since the angular velocity of a deflector is constant, the scanned face is not scanned at a constant speed so that it is necessary to take measures for performing the scanning operation at the constant speed. An f.theta. lens system is a lens system for optically realizing the scanning operation of the scanned face at the constant speed. This lens system has an f.theta. function in which the height of an image formed by the incident light beam becomes f.theta. when an angle between the incident light beam and a lens optical axis is .theta. and the focal distance of the lens is f.
As a method for solving the problem of the reflecting face inclination, there is a known method in which a lens system disposed between the rotary polygon mirror and the scanned face is set to an anamorphic optical system, and a reflecting position of the rotary polygon mirror and the scanned face are connected to each other in a conjugate relation with respect to a secondary scanning direction.
Japanese Patent Application Laying Open (KOKAI) No. 59-147316 discloses an f.theta. lens system set as the anamorphic optical system to solve the problems about the scanning operation at a constant speed and the inclination of the reflecting face. This lens system has a large deflection angle, but it is not necessarily sufficient to correct a curve of an image face in the main and secondary scanning directions. Further, the diameter of an image forming spot on the scanned face is greatly varied depending on the scanning position so that it is difficult to realize the optically scanning operation of high density.
Further, when a normal f.tan .theta. lens is used, the scanned face is not scanned by the deflected light beam at a constant speed since the angular velocity of the rotary polygon mirror is constant. Japanese Patent Application Laying Open (KOKAI) No. 63-19617 discloses an f.theta. lens system set as the anamorphic optical system and using two lenses to solve the problems about the scanning operation at a constant speed and the inclination of the reflecting face. However, in this f.theta. lens system, it is not necessarily sufficient to correct a curvature of a field. Further, the diameter of an image forming spot on the scanned face is greatly varied depending on the scanning position so that it is difficult to realize the optically scanning operation of high density. Japanese Patent Application Laying Open (KOKAI) No. 61-120112 discloses an f.theta. lens system using a so-called saddle-type toric face and constructed by two lenses to preferably correct the field curvature. However, in this f.theta. lens system, a lot of aspherical faces are used and thereby the cost of the system is expensive. Further, the saddle-type toric face is used as a lens face of an elongated lens so that it is difficult to make the optical scanner compact.
Further, in this f.theta. lens system, the aspherical faces are used, it is difficult to process this lens system.